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Discuss How to shield a live wire at 240V ? in the Electrical Forum area at ElectriciansForums.net

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I have a electronic board with leds (made by me) that is driving a relay. When that relay is closing it’s contacts, is actually switching the 240V for the light bulb switch in my room. This board is quite long, like 50cm (half a meter). The relay is in the left corner, and the live wires from it goes in behind the board to the hole in the wall for the mains switch. I also have a mild steel sheet behind my board that is grounded. It is shielding the interference of the 50Hz from the live wires from the wall to my sensitive circuit. But even If I have this grounded metal shield behind my board, the live wires from the relay are still affecting my entire circuit board, keeping it ON all the time. If I am disconecting the live wires from the relay, the board is functioning very well. Another IF, is if I disconnect the ground from the metal shield, the circuit board goes nuts. So the shield is doing it's job fine, but only for the live wires inside the wall !!! But not for the wires from the relay to the live switch.
- I want a way to shield these wires !
Thank you !
 
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Lucien Nunes

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There seems to be a problem with your electronic design that is making the circuit too sensitive to interference. Let's see the circuit.

Also, you give your location as New York but speak of 240V 50Hz lighting. Where are you?
 
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could you fit the relay in the switch box so it's behind the sheet metal shield?
@telectrix - Thank you for your good answer!
Yes, that is my plan as well. But until I will make this operation, I was thinking maybe I can shield those wires, with all the components on my board as they are now, without any other modification. I will modify it in the end, exactly as you say, but... I wanted a more convenient way.

There seems to be a problem with your electronic design that is making the circuit too sensitive to interference. Let's see the circuit.

Also, you give your location as New York but speak of 240V 50Hz lighting. Where are you?
- Thank you. Yes, you guessed right. The board contains a presence sensing module, meaning, when my hand is in vicinity to that module, it will output high +5V to the next module. It is very sensitive module indeed. And without the metal shield behind it, the entire board will just stay ON all the time, having erroneous behavior. Exactly because of this sensibility of it. And my guess is that 50Hz from the wall wires are not interfering with only the sensing module, but with the entire circuit board, which is quite large and spread. And yes, I like New York but im in Europe.

So tell me straight. Those wires can not be shielded at all ? I am discussing this problem on other forum but for electronics and not electrical like this one here and they got silent about this part. So I figure out to ask more around.
Thank you so much for the direct answers so far !
 

Lucien Nunes

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Tel has the right answer. Put the relay in a box, separate it from the main PCB and just run the DC coil circuit to the box. Ensure that the 230V AC and ELV DC circuits are properly insulated from one another, so that in the event of accidental damage, the DC circuit is not likely to become live. If you are making this part of your electrical installation, there are constructional standards that you should adhere to, in order not to invalidate the electrical installation certificate on your house.

But, as I said earlier, an electronic device that doesn't work correctly near ordinary domestic power cables is not a practical device because that's a very severe limitation on its usage envelope, like a phone that doesn't work in the dark or an electric toothbrush that must not be used in the bathroom. A significant amount of R&D is often needed to overcome issues of this kind, over and above what is needed to make it carry out its basic functions.
 

pc1966

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So tell me straight. Those wires can not be shielded at all ? I am discussing this problem on other forum but for electronics and not electrical like this one here and they got silent about this part.
You can get shielded power cables, examples include SWA (steel wired armour) and Flexishield or similar cables to BS 8436 (UK standard) or IS 273 (Republic of Ireland standard). But they are not commonly used for indoor wiring.

As Lucian explained above, you have to design something to work in the typical environment it will be used. Working is a restricted environment is fine for laboratory and other specialised cases when you can arrange it, but not as a general product for home use.
 
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You can get shielded power cables, examples include SWA (steel wired armour) and Flexishield or similar cables to BS 8436 (UK standard) or IS 273 (Republic of Ireland standard). But they are not commonly used for indoor wiring.
Then... can you explain why the metal shield is shielding the live wires in the wall and not the ones outside the wall? By the way, the wall wires are thick and my outside wires are thin in diameter. Probably it will be the same intensity of the field on both. I imagine. I really don't know.
As Lucian explained above, you have to design something to work in the typical environment it will be used. Working is a restricted environment is fine for laboratory and other specialised cases when you can arrange it, but not as a general product for home use.
Oh, this is just a personal project. Not a public or commercial one. Don't worry. It's destination is only to my room use. And is very experimental as well, with exposed circuits and components. I would not trust to mount it on other places than mine. Haha. But is a good point you bring and and a fear for "some" out there doing too weird things.Interesting indeed.
 
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You can get shielded power cables, examples include SWA (steel wired armour) and Flexishield or similar cables to BS 8436 (UK standard) or IS 273 (Republic of Ireland standard). But they are not commonly used for indoor wiring.
Excellent answer ! I mean I know the general term of coaxial, triaxial, and another 2. But that is just theory that I happen to read about in my research. And actually plan to shield my cable in a very short time, just as a test. Just to see if it will affect in any way. It will be awesome to tell me now, before doing anything if its worth to try or not.
 

Lucien Nunes

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You can use a shielded cable for the power but that is not dealing with the root cause of the problem. It's a kludge like attaching a light to the phone that won't work in the dark.

If you show the circuit, we might be able to suggest some tweaks.
 
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